rs6000: Rework vector integer comparison in rs6000_emit_vector_compare - p1

The current handlings in rs6000_emit_vector_compare is a bit
complicated to me, especially after we emit vector float
comparison insn with the given code directly.  So it's better
to refactor the handlings of vector integer comparison here.

This is part 1, it's to remove the helper function
rs6000_emit_vector_compare_inner and move the logics into
rs6000_emit_vector_compare.  This patch doesn't introduce any
functionality change.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_emit_vector_compare_inner): Remove.
	(rs6000_emit_vector_compare): Emit rtx comparison for operators EQ/
	GT/GTU directly.